Details concerning the process of reverse transcription in mammalian cells newly infected with RNA tumor viruses are essentially unknown. The objectives of this research are to understand the process of RNA to DNA transcription as it occurs in RNA tumor virus-infected cells, with particular emphasis on determining the function and mode of action of RNase H in the process. These objectives will be achieved: 1) by determining the structural, catalytic, and antigenic properties and intravirion location of RNA-directed DNA polymerase-associated RNase H and free RNase H activities purified from mammalian RNA tumor virions, 2) by the use of model substrates synthesized in vitro to elucidate the mode of action of viral RNase H, and 3) by the isolation of a "reverse transcription complex" from newly infected mammalian cells and characterization of the enzymes and viral related nucleic acids associated with and synthesized by the complex.